1. Cse 344
March 2nd – E/r diagrams

2. Administrivia All HWs Out
For HW8, if you need additional Azure credit, send me an
Transactions, starting today
Only one tag for HW8!

3. Database Design What it is:
Starting from scratch, design the database schema: relation, attributes, keys, foreign keys, constraints etc
Why it’s hard
The database will be in operation for a very long time (years). Updating the schema while in production is very expensive (why?)

4. 3. Design Principles What’s wrong? Purchase Product Person President
Country
Design does not reflect reality
Person
Moral: Be faithful to the specifications of the application!

5. Design Principles: What’s Wrong?
date
Product
Purchase
Store
We have redundancy: repeat address of person for each purchase. More next lecture.
Moral: pick the right
kind of entities.
personAddr
personName

6. Design Principles: What’s Wrong?
date
Dates
Product
Purchase
Store
Moral: don’t
complicate life more
than it already is.
Person

7. Entity Set to Relation Product(prod-ID, category, price) prod-ID
Gizmo55
Camera
99.99
Pokemn19
Toy
29.99

8. N-N Relationships to Relations
prod-ID
cust-ID
date
name
date
Shipment
Shipping-Co
Orders
address
Represent this in relations

9. N-N Relationships to Relations
prod-ID
cust-ID
date
name
date
Shipment
Shipping-Co
Orders
Orders(prod-ID,cust-ID, date)
Shipment(prod-ID,cust-ID, name, date) Shipping-Co(name, address)
address
prod-ID
cust-ID
name
date
Gizmo55
Joe12
UPS
4/10/2011
FEDEX
4/9/2011

10. N-1 Relationships to Relations
prod-ID
cust-ID
date
name
date
Shipment
Shipping-Co
Orders
address
Represent this in relations

11. N-1 Relationships to Relations
prod-ID
cust-ID
date
name
date
Shipment
Shipping-Co
Orders
address
Orders(prod-ID,cust-ID, date1, name, date2) Shipping-Co(name, address)
Remember: no separate relations for many-one relationship

12. Multi-way Relationships to Relations
address
name
Product
Purchase
price
Store
prod-ID
Person
Try this at home!
ssn
name
Purchase(prod-ID, ssn, name)

13. Modeling Subclasses Some objects in a class may be special
define a new class
better: define a subclass
Products
Software
products
Educational
products
So --- we define subclasses in E/R

14. Modeling Subclasses name category price Product isa isa
Software Product
Educational Product
platforms
Age Group

15. Modeling Subclasses Name Price Category Gizmo 99 gadget Camera 49
Product
Name
Price
Category
Gizmo 99
gadget
Camera 49
photo
Toy 39
Product
name
category
price
isa
Educational Product
Software Product
Age Group
platforms
Sw.Product
Name
platforms
Gizmo
unix
There are different ways to convert from subclasses to relations (remember Phil B’s example)
Ed.Product
Name
Age Group
Gizmo
toddler
Toy
retired
Other ways to convert are possible

16. Modeling Union Types with Subclasses
FurniturePiece
Person
Company
Say: each piece of furniture is owned either by a person or by a company

17. Modeling Union Types with Subclasses
Say: each piece of furniture is owned either by a person or by a company
Solution 1. Acceptable but imperfect (What’s wrong ?)
FurniturePiece
Person
Company
ownedByPerson
ownedByComp.
Problem: Can still have the same furniture owned by both a person and a company simultaneously

18. Modeling Union Types with Subclasses
Solution 2: better, more laborious
Owner
isa
isa
ownedBy
Person
Company
FurniturePiece

19. Weak Entity Sets Entity sets are weak when their key comes from other
classes to which they are related.
affiliation
Team
University
sport
number
name
Weak entity sets: an entity set’s key is composed of attributes, some or all of which belong to another entity
Don’t need to construct relation for affiliation since team gets its key from university
Team(sport, number, universityName) University(name)

20. What Are the Keys of R ? A B R H S T C W U V V L F K E Q G Z D
The slide is a quick exercise for understanding weak entity sets. Ask the class: “what are the keys of R?”
Since R is weak, it must include all keys that we can reaching following weak relationships.
Answer: R.key = A, C, D, H, F, E, K
Notice that we do not include L, or G, because the relationship S->U is not weak. W U V V L F K E Q G Z D

21. Integrity Constraints Motivation
An integrity constraint is a condition specified on a database schema that restricts the data that can be stored in an instance of the database.
ICs help prevent entry of incorrect information
How? DBMS enforces integrity constraints
Allows only legal database instances (i.e., those that satisfy all constraints) to exist
Ensures that all necessary checks are always performed and avoids duplicating the verification logic in each application

22. Constraints in E/R Diagrams
Finding constraints is part of the modeling process.
Commonly used constraints:
Keys: social security number uniquely identifies a person.
Single-value constraints: a person can have only one father.
Referential integrity constraints: if you work for a company, it
must exist in the database.
Other constraints: peoples’ ages are between 0 and 150.

23. Keys in E/R Diagrams name category Underline: price No formal way
to specify multiple
keys in E/R diagrams
Product
Person
name
ssn
address

24. Single Value Constraints
makes
vs.
makes

25. Referential Integrity Constraints
makes
Product
Company
Each product made by at most one company.
Some products made by no company
Round arrow: every product must be owned by exactly one company
NOTE: Foreign keys are allowed to be NULL, so the top one can still correspond to a FK constraint.
makes
Product
Company
Each product made by exactly one company.

26. Other Constraints makes <100 Product Company
Q: What does this mean ?
A: A Company entity cannot be connected
by relationship to more than 99 Product entities

27. Constraints in SQL Constraints in SQL: Keys, foreign keys
Attribute-level constraints
Tuple-level constraints
Global constraints: assertions
The more complex the constraint, the harder it is to check and to enforce
simplest
Most complex

28. Key Constraints OR: Product(name, category) CREATE TABLE Product (
name CHAR(30) PRIMARY KEY,
category VARCHAR(20))
We have already learned about this earlier
CREATE TABLE Product (
name CHAR(30),
category VARCHAR(20),
PRIMARY KEY (name))
OR:

29. Keys with Multiple Attributes
Product(name, category, price)
CREATE TABLE Product (
name CHAR(30),
category VARCHAR(20),
price INT,
PRIMARY KEY (name, category))
Name
Category
Price
Gizmo
Gadget 10
Camera
Photo 20 30 40

30. Other Keys CREATE TABLE Product ( productID CHAR(10), name CHAR(30),
category VARCHAR(20),
price INT, PRIMARY KEY (productID),
UNIQUE (name, category))
Recall that primary key is the order for which rows are laid out on the disk!
There is at most one PRIMARY KEY; there can be many UNIQUE

31. Foreign Key Constraints
Referential integrity constraints
CREATE TABLE Purchase (
prodName CHAR(30)
REFERENCES Product(name),
date DATETIME)
May write just Product if name is PK
prodName is a foreign key to Product(name) name must be a key in Product

32. Foreign Key Constraints
Example with multi-attribute primary key (name, category) must be a KEY in Product
CREATE TABLE Purchase (
prodName CHAR(30),
category VARCHAR(20),
date DATETIME,
FOREIGN KEY (prodName, category)
REFERENCES Product(name, category)

33. What happens when data changes?
Types of updates:
In Purchase: insert/update
In Product: delete/update
Product
Purchase
Name
Category
Gizmo
gadget
Camera
Photo
OneClick
ProdName
Store
Gizmo
Wiz
Camera
Ritz

34. What happens when data changes?
SQL has three policies for maintaining referential integrity:
NO ACTION reject violating modifications (default)
CASCADE after delete/update do delete/update
SET NULL set foreign-key field to NULL
SET DEFAULT set foreign-key field to default value
need to be declared with column, e.g., CREATE TABLE Product (pid INT DEFAULT 42)

35. Maintaining Referential Integrity
CREATE TABLE Purchase (
prodName CHAR(30),
category VARCHAR(20),
date DATETIME,
FOREIGN KEY (prodName, category)
REFERENCES Product(name, category)
ON UPDATE CASCADE
ON DELETE SET NULL )
Product
Purchase
You can possibly end up in a cycle this way. (need to temporarily drop the constraints in order to update rows)
Name
Category
Gizmo
gadget
Camera
Photo
OneClick
ProdName
Category
Gizmo
Snap
Camera
EasyShoot

36. Constraints on Attributes and Tuples
Constraints on attributes: NOT NULL -- obvious meaning... CHECK condition -- any condition !
Constraints on tuples CHECK condition

37. Constraints on Attributes and Tuples
CREATE TABLE R ( A int NOT NULL,
B int CHECK (B > 50 and B < 100),
C varchar(20),
D int,
CHECK (C >= 'd' or D > 0))
Tuple-based constraints are checked more often than attribute-based constraints

38. Constraints on Attributes and Tuples
CREATE TABLE Product (
productID CHAR(10),
name CHAR(30),
category VARCHAR(20),
price INT CHECK (price > 0), PRIMARY KEY (productID),
UNIQUE (name, category))

39. Constraints on Attributes and Tuples
What is the difference from Foreign-Key ?
What does this constraint do?
CREATE TABLE Purchase (
prodName CHAR(30)
CHECK (prodName IN (SELECT Product.name FROM Product),
date DATETIME NOT NULL)
Constraints on attributes are only checked when the value of the attribute changes (so they could potentially be violated by other changes).
So, unlike a FK, if Product changes, this check won’t catch the problem.

40. General Assertions CREATE ASSERTION myAssert CHECK (NOT EXISTS(
SELECT Product.name FROM Product, Purchase WHERE Product.name = Purchase.prodName GROUP BY Product.name HAVING count(*) > 200) )
The DBMS would have to deduce whether a change can affect the truthfulness of an assertion.
But most DBMSs do not implement assertions
Because it is hard to support them efficiently
Instead, they provide triggers

41. Class Overview Unit 1: Intro
Unit 2: Relational Data Models and Query Languages
Unit 3: Non-relational data
Unit 4: RDMBS internals and query optimization
Unit 5: Parallel query processing
Unit 6: DBMS usability, conceptual design
Unit 7: Transactions
Locking and schedules
Writing DB applications
For each item, say why we are learning about it.

42. Transactions We use database transactions everyday Bank $$$ transfers
Online shopping
Signing up for classes
For this class, a transaction is a series of DB queries
Read / Write / Update / Delete / Insert
Unit of work issued by a user that is independent from others

43. Challenges Want to execute many apps concurrently
All these apps read and write data to the same DB
Simple solution: only serve one app at a time
What’s the problem?
Want: multiple operations to be executed atomically over the same DBMS

44. What can go wrong? Manager: balance budgets among projects
Remove $10k from project A
Add $7k to project B
Add $3k to project C
CEO: check company’s total balance
SELECT SUM(money) FROM budget;
This is called a dirty / inconsistent read aka a WRITE-READ conflict

45. What can go wrong?
App 1: SELECT inventory FROM products WHERE pid = 1 App 2: UPDATE products SET inventory = 0 WHERE pid = 1 App 1: SELECT inventory * price FROM products WHERE pid = 1 This is known as an unrepeatable read aka READ-WRITE conflict

46. Account 1 = $100 Account 2 = $100 Total = $200
What can go wrong?
Account 1 = $100 Account 2 = $100 Total = $200
App 1:
Set Account 1 = $200
Set Account 2 = $0
App 2:
Set Account 2 = $200
Set Account 1 = $0
At the end:
Total = $200
App 1: Set Account 1 = $200
App 2: Set Account 2 = $200
App 1: Set Account 2 = $0
App 2: Set Account 1 = $0
At the end:
Total = $0
This is called the lost update aka WRITE-WRITE conflict

47. What can go wrong? Lesson:
Paying for Tuition (Underwater Basket Weaving)
Fill up form with your mailing address
Put in debit card number (because you don’t trust the gov’t)
Click submit
Screen shows money deducted from your account
[Your browser crashes]
Lesson:
Changes to the database should be ALL or NOTHING

48. If BEGIN… missing, then TXN consists of a single instruction
Transactions
Collection of statements that are executed atomically (logically speaking)
BEGIN TRANSACTION [SQL statements]
COMMIT or ROLLBACK (=ABORT)
[single SQL statement]
If BEGIN… missing, then TXN consists of a single instruction

49. Know your transactions: ACID
Atomic
State shows either all the effects of txn, or none of them
Consistent
Txn moves from a DBMS state where integrity holds, to another where integrity holds
remember integrity constraints?
Isolated
Effect of txns is the same as txns running one after another (i.e., looks like batch mode)
Durable
Once a txn has committed, its effects remain in the database

50. Atomic
Definition: A transaction is ATOMIC if all its updates must happen or not at all.
Example: move $100 from A to B
UPDATE accounts SET bal = bal – 100 WHERE acct = A;
UPDATE accounts SET bal = bal WHERE acct = B;
BEGIN TRANSACTION; UPDATE accounts SET bal = bal – 100 WHERE acct = A; UPDATE accounts SET bal = bal WHERE acct = B; COMMIT;

51. Isolated
Definition:
An execution ensures that transactions are isolated, if the effect of each transaction is as if it were the only transaction running on the system.

52. Consistent
Recall: integrity constraints govern how values in tables are related to each other
Can be enforced by the DBMS, or ensured by the app
How consistency is achieved by the app:
App programmer ensures that txns only takes a consistent DB state to another consistent state
DB makes sure that txns are executed atomically
Can defer checking the validity of constraints until the end of a transaction

53. Durable
A transaction is durable if its effects continue to exist after the transaction and even after the program has terminated
How?
By writing to disk!
More in 444

54. Rollback transactions
If the app gets to a state where it cannot complete the transaction successfully, execute ROLLBACK The DB returns to the state prior to the transaction What are examples of such program states?

55. ACID Atomic Consistent Isolated Durable
Again: by default each statement is its own txn
Unless auto-commit is off then each statement starts a new txn